The present invention relates to hardening metals or metal alloys, and more particularly to hardening AerMet® 100 alloy for uses such as gears within a rotary-wing aircraft gearbox.
Certain compositions of steel, such as Pyrowear® 53 and 9310, have been used for gears or other applications requiring high strength and fatigue resistance. Pyrowear® 53 and 9310 typically have a strength of 180-200 ksi and a hardness of 30-45 HRc. To increase fatigue resistance, the gears are carburized to produce a case that surrounds a less hard core. For example, gears made of Pyrowear® 53 or 9310 are heated to an austenizing temperature of 1650° F. in a 0.8% carbon atmosphere. The carbon diffuses into solid austenite solution. Upon quenching, the austenite forms high carbon martensite having a surface carbon level around 0.8% to 1.0%, which hardens the surface.
Although Pyrowear® 53 and 9310 are useful for gears and other applications, there is a need for even stronger and more fatigue-resistant gears. AerMet® 100 is an alloy developed by Carpenter Technology based on the composition of US Air Force Alloy 1410. When processed per CarTech specified directions or per AMS 6532 specification, AerMet® 100 develops an ultimate strength of 280 ksi, a fracture toughness value of 115 ksi·inch1/2 and a hardness of 53 HRC. The strength and toughness combination make AerMet® 100 attractive for use in gears, however, AerMet® 100 lacks the desired surface hardness.
One proposed solution to enable use of AerMet® 100 in gears is to carburize the AerMet® 100 using the conventional carburization process that is used for Pyrowear® 53 and 9310. However, instead of forming hardened high carbon martensite, AerMet® 100 forms undesirable microstructures that prevent use of AerMet® 100 in gears and other applications.
Accordingly, it is desirable to provide a method of hardening AerMet® 100 for use in gears and other applications, while avoiding the shortcomings and drawbacks of the prior art.